This invention relates to an apparatus for detecting the passage of multiple superposed sheets along a feed path. The invention has application, for example, to an apparatus for detecting the passage of superposed currency notes in a cash dispensing mechanism of an automated teller machine (ATM).
In a cash dispensing mechanism, it is important to provide a simple and reliable means for detecting when a currency note has become superposed on another in a path of travel from a currency supply means to a note exit slot, since such superpositioning may produce an undesirable result such as the dispensing of an excessive amount of money. For convenience, two or more sheets or notes which have become disposed in a superposed relationship will hereinafter be referred to as a multiple sheet or a multiple note.
One known type of apparatus for detecting the passage of multiple notes along a feed path employs a note thickness sensing mechanism through which notes are fed in operation and which incorporates a gauging roller. In the event of a multiple note (or an excess thickness note) passing through the sensing mechanism, the axis of the gauging roller is displaced by an amount such that a note rejecting means is actuated, actuation of the rejecting means causing the notes or note to be diverted into a reject hopper. A problem experienced with known apparatuses of this type is that such apparatus may not distinguish between multiple notes and a single note having a localized increase in thickness, brought about for example by a crease or fold in the note or by the attachment thereto of extraneous matter such as adhesive tape. As a result, there is a tendency for such apparatus to reject an excessive number of notes. The use of such apparatus in a cash dispensing mechanism of an ATM would tend to increase maintenance costs, since the rejection of an excessive number of notes would decrease the period of time between successive replenishments of the machine with currency notes.
An apparatus which is intended to overcome the aforementioned problem is disclosed for example in U.K. Patent Application No. 2,001,038A. A thickness sensor including a pair of gauging rollers is arranged to gauge a portion of a currency note and to generate a digital signal in response to note thickness. For example, a logic 1 signal in response to a multiple note thickness and a logic 0 signal in response to a single note thickness may be used. The digital output of the thickness sensor is applied to an integrator circuit which integrates this output over substantially the entire length of the gauged portion of the note. The output of the integrator circuit is compared with a reference signal in order to determine if the gauged note is a multiple note or a single note. A disadvantage of this known apparatus is that critical adjustment of the thickness sensor is required if the apparatus is to operate satisfactorily.